Methods and apparatus for watermarking digitally printed documents

Abstract

A form of stochastic random distribution is used to print an invisible watermark using a typical, e.g., 600 dpi, printer. The watermark contains such information as “Do Not Copy,” or other such information. Although the watermark is not visible on a first generation version of a document that is printed, various techniques are used to print the watermark in such a way that it becomes readily apparent when the first generation document is photocopied. The preferred embodiment of the invention is concerned with a technique for improving such techniques, for example so that gray levels are improved to address variations from printer to printer. Another preferred embodiment of the invention is also concerned with a technique for performing error diffusion watermarking without the need for performing a calibration step.

1. A method for printing a watermark in a document, the method comprising:
 generating foreground data comprising the watermark; 
 generating background data; 
 using photocopy reproducible screening in connection with the background data: and 
 using photocopy irreproducible screening in connection with the foreground data, wherein at least one of the photocopy reproducible and photocopy irreproducible screenings is accomplished using a stochastic screen, and wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both the foreground image and the background image is photocopied. 
 
     
     
       2. Apparatus for printing a watermark in a document that comprises foreground data comprising the watermark, and background data, the apparatus comprising:
 a first screen for providing a photocopy reproducible version of the background data: and 
 a second screen for providing a photocopy irreproducible version of the foreground data, wherein at least one of the first and second screens is a stochastic screen and wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both the foreground image and the background image is photocopied. 
 
     
     
       3. A method for printing a watermark in a document, the method comprising:
 processing foreground data with a foreground screen, the foreground screen producing a photocopy irreproducible image; 
 processing background data with a background screen, the background screen producing a photocopy reproducible image where at least one of the foreground and background screens is a stochastic screen; and 
 compositing the line-screened foreground and background data to produce a combined image for printing on a printer, wherein there is a variance between a foreground image and a background image in perceptual intensity when a printed document comprising both the foreground image and the background image is photocopied.